Add powsybl-cgmes-gl repo to MAINTAINERS.md

[geofjamg]

Signed-off-by: Geoffroy Jamgotchian geoffroy.jamgotchian@rte-france.com

Please check if the PR fulfills these requirements (please use '[x]' to check the checkboxes, or submit the PR and then click the checkboxes)

• [x] The commit message follows our guidelines
• [ ] Tests for the changes have been added (for bug fixes / features)
• [ ] Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...) docs update

What is the new behavior (if this is a feature change)? powsybl-cgmes-gl repo has been added to MAINTAINERS.md